The invention relates to a computed tomography apparatus which includes a radiation source which emits a conical radiation beam and moves along a helical path relative to an examination zone or an object present therein. In computed tomography apparatus of this kind (for example, as known from U.S. application Ser. No. 09/380972 (PHQ 98-020)), the absorption cannot be reconstructed without artefacts in the zones that are scanned by the conical radiation beam at the beginning or the end of a CT examination, because the radiation source irradiates these zones from a limited angular range only.
The ratio of the zone that is exposed to radiation to the zone that can be reconstructed without artefacts is less favorable as the angle of aperture of the radiation beam (in two mutually perpendicular planes) is larger. Thus, only a part of the radiation can be used for the CT imaging; in the case of medical examinations this leads to an unnecessarily high radiation load for the patient. When, moreover, the cylindrical examination zone is allowed to begin and end by way of the first and the last layer, respectively, that can be fully reconstructed and extends perpendicularly to the cylindrical examination zone, the circumstances become entirely unfavorable.
Therefore, it is an object of the present invention to improve these conditions. This object is achieved in accordance with the invention by means of a computed tomography apparatus which includes:
a scanning unit which includes a radiation source and a detector unit which is connected thereto in order to detect a conical radiation beam, emitted by the radiation source, after its passage through an examination zone (13), or an object arranged on a table top situated therein, and to generate corresponding CT data,
a drive device (2, 5) for producing a relative motion in the form of a helix, including a rotation about an axis of rotation (14) and a displacement parallel to the axis of rotation, between the scanning unit (S, 16) and the examination zone (13) or the object,
a reconstruction unit for reconstructing a three-dimensional CT image from the measuring values produced by the detector unit, and
a control unit for controlling the radiation source and the drive device in such a manner that emission of the radiation beam by the radiation source commences in a start position such and/or ends in a stop position such that the zones which, at the beginning and/or the end of the emission, are irradiated by the radiation beam only to an extent that is not adequate for reconstruction, are larger underneath the table top, in the direction of the axis of rotation, than above the table top.
The invention is based on the recognition of the fact that the cylindrical part of the examination zone for which complete reconstruction is possible is bounded on both sides by end faces which do not extend perpendicularly to the axis of rotation and are not flat either. When the start position and/or the stop position are suitably chosen, it can be achieved that (measured in the direction of the axis of rotation) the part of the examination zone that can be reconstructed is larger above the table than below the table (or that the part that cannot be reconstructed is larger underneath the table than above the table).
The zone that can be imaged by way of layers extending perpendicularly to the axis of rotation can then be selected in conformity with the dimensions of the zone that can be reconstructed above the table. Granted, in that case the zone situated underneath the table can no longer be reconstructed in the layers situated at the edge; however, this limitation is irrelevant, because the object, and hence the region of interest for the diagnosis, are situated above the table.
The optimum start and stop positions of the radiation source are usually situated above a horizontal plane that contains the axis of rotation. The exact start and stop positions, however, are also dependent on how the measuring values are acquired and on how a three-dimensional CT image is reconstructed from these measuring values. For example, for the CT method disclosed in the previously mentioned publication the optimum start position is obtained in conformity with claim 2 and the stop position in conformity with claim 3.
Because in the case of the known methods the end faces of the zone that can be completely reconstructed are not flat, the optimum start position is also dependent on the level at which the table punctures said end faces. When the table is situated, for example, below the axis of rotation during a CT examination, the start position and the stop position must be shifted by the same angular amount in the direction of revolution. claim 4 describes a version which is suitable for automatically adapting the start position and the stop position to the vertical position of the table.
As has already been stated, the zones that are situated underneath the table top can no longer be completely reconstructed in the layers situated at the ends, so that artefacts occur in these zones. These artefacts can be avoided in conformity with claim 5 where the user is given the normal image impression. The zones that cannot be fully reconstructed underneath the table, however, could also be suitably marked (for example, colored) during display so that the user is not falsely given the impression of a complete reconstruction in this zone.
The invention will be described in detail hereinafter with reference to the drawings. Therein:
FIG. 1 is a diagrammatic perspective view of a computed tomography apparatus in accordance with the invention,
FIG. 2 is a cross-sectional view of such a computed tomography apparatus,
FIG. 3 shows the part of the examination zone that can be reconstructed without using the invention,
FIGS. 4a and 4b show the start position and the stop position, respectively, of the radiation source,
FIG. 5 shows the part of the examination zone that can be reconstructed in accordance with the invention, and
FIG. 6a and FIG. 6b show the end faces of the zone that can be reconstructed.